Isolation zeolites, shape selectivity

Following the discovery of TS-1 [125], a titanium-substituted MFl, the use of zeolitic materials for oxidation increased significantly. The presence of the Ti atom in the framework of a zeolite structure provides a site-isolated Ti center, a situation not possible with other Ti-containing materials while also allowing shape-selective oxidations. The combination of the two effects gives highly active and selective oxidation reactions [126]. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

PCPs with well-defined pores and surface-isolated Lewis acid sites could potentially serve as size- or shape-selective heterogeneous catalysts, in a similar manner to zeolites.33 43 161-164 The two-dimensional PCP, [Cd(4,4 -bpy)2(H20)2] 2N03 4H20 , was the first example that showed catalytic properties for the cyanosilylation of aldehydes.33 Experimental data in the case of cyanosilylation of imines, which is also performed by the same compound, led to the conclusion that hydrophobic grid cavities bind to the substrate very efficiently to promote a rapid reaction, and that the heterogeneous reaction involves the selective activation of the imino nitrogen by the weak Lewis acid Cdn center.161 In this polymer, the NO3" anions exist in a coordination-free state. This situation contributes to increasing the Lewis acidity of the Cdn centers. 

H. van Bekkum et al. (17) reported that the alpha-pinene oxide 9 can be succesfully converted to campholenic aldehyde 10 (Eq. 15.2.5) in the presence of a BEA-zeolite. Ti-BEA proves to be an excellent catalyst for the rearrangement of a-pinene oxide to campholenic aldehyde in both the liquid and vapor phase. This is mainly attributed to the presence of isolated, well-dispersed titanium sites in a Bronsted-acid-free silica matrix. Furthermore, the unique molecularsized pore structure of the zeolite may enhance selectivity by shape-selectivity. 

Another option that sometimes enables immobilization of isolated metal ions stable to leaching, and avoidance of the formation of oligomers, is the synthesis of zeolites or zeotypes containing isolated metal ions in framework positions. In these the oxidation properties of the metal atoms are associated with the main characteristics of zeolites which involve shape-selective effects and unique adsorption properties which can be tuned in terms of their hydrophobicity-hydrophi-licity, enabling selection of the proportions of reactants with different polarities that will be adsorbed in the pores. Researchers at ENI succeeded in introducing Ti into silicalite producing the TS-1 redox molecular sieve oxidation catalyst [64]. TS-1 has an MFI structure formed by a bidimensional system of channels with 0.53 nm X 0.56 nm and 0.51 nm X 0.51 nm pore dimensions. The incorporation of Ti into the framework has been demonstrated by use of several techniques-XRD, UV-visible spectrophotometry, EXAFS-XANES a good review has been published by Vayssilov [65]. 

The use of solid catalysts in halogenation processes will avoid corrosion and disposal problems. Work-up procedures to isolate and recover the desired product will also be easier leading to simpler and cleaner process routes. In addition, the use of zeolites as the solid catalysts in nuclear aromatic halogenations might lead to enhanced yields of the para isomer, because of the shape selectivity of the zeolite. 

In the Fries liquid-phase rearrangement of PA carried out in the presence of ZSM-5(41) zeolite at 170°C, a good shape selectivity (ortho/para = 0.17) can be reached after 24 h by keeping modest the conversion of the starting ester (- 20%). When the reaction is repeated over Nu-10 zeolite, a 1 1 mixture of ortho- and para-products is isolated this different behavior can be ascribed to the smaller pore dimension of Nu-10 zeolite, which hampers the entrance of the reagents producing both isomers. 

For the acid catalysed conversion of hydrocarbons, the reaction mechanisms in absence of sterical hinderance are rather well understood, so that molecular shape-selective effects exerted by constrained environments can be isolated [8,9]. Shape-selective catalysis is also possible when other than acid functions are confined to the intracrystalline void volumes of zeolite crystals, e.g. metal [10,11], bifunctional [12] and basic functions [13]. Nowadays, catalysis on zeolites with organic substrates containing heteroatoms receives much attention. Molecular shape-selectivity seems to be superimposed on electronic factors determining the selectivities [14,15]. 

Several new types of anionic tetrahedron complexes were discovered during the last years an Isolated complex, built of 48 Sl,0 tetrahedra, pyrogroups [P2O7] In a phosphate mineral, a 18-membered [SiOs] rings, a [PO3] chain with 16 tetrahedra In the period, an interrupted [81205] framework, etc. The new scientific ideas on the structures of the siiicates and their analogues stimulate their appiications in different technologies (ex. shape selective catalysis with zeolites). 

Thus, LDPE complements zeolites, which have been described earlier to serve a similar role in isolating molecules for intramolecular cyclizations [68], By contrast, the reaction cavities of zeolites have fixed volumes and shapes and a very hard wall [12-15], Each medium has its relative merits now, it is possible to select the one that is more compatible with a particular reactant and cyclization reaction. 

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